For some plated steel sheets, a chemical conversion film is formed that contains an organic resin in order to prevent galling at the time of formation (e.g., see Patent Literatures 1 and 2; hereinafter referred to as “PTL 1” and “PTL 2”). PTL 1 and 2 disclose that a chemical conversion film which contains an organic resin such as a urethane resin is formed on a surface of a zinc-based plated steel sheet. Coating the surface of the plated steel sheet in such a manner with a chemical conversion film that contains an organic resin not only allows for galling resistance to be improved, but also allows for corrosion resistance, tarnish resistance and/or the like to be improved.
On the other hand, fluororesins, which exhibit excellent weather resistance, are sometimes used as the organic resin constituting the chemical conversion film in order to improve the weather resistance of the chemically treated plated steel sheet. When using fluororesins for improving weather resistance in such a manner, organic solvent-based fluororesin compositions are frequently used. However, such organic solvent-based fluororesin compositions have the drawbacks of being fire hazards, toxicity hazards, air pollutants, and/or the like.
Moreover, various water-based fluororesin compositions are also provided (e.g., see Patent Literature 3; hereinafter referred to as “PTL 3”). However, such water-based fluororesin compositions all require baking at high temperatures (e.g., 180-230° C.; see PTL 3). Such a high-temperature baking is not feasible at painting sites (where air-dry resins are typically used), and is disadvantageous even in production lines where heat-drying is mainly employed upon formation of coatings.
In addition, in order to solve the problem pertinent in water-based fluororesins, a curable moiety (organic functional group) is introduced, and water-based fluororesin compositions are thereby provided from which a film can be formed even at low temperatures (e.g., see Patent Literature 4; hereinafter referred to as “PTL 4”). However, in the case of a cured film in which organic functional group have been reacted, weather resistance is preferentially deteriorated from the cured portions, and therefore the film becomes porous and water resistance decreases. Moreover, even when a surface treatment has been implemented using an epoxy resin, urethane resin or the like in order to improve adhesion, the organic resin preferentially deteriorated weather resistance, and dramatically reduces film adhesion.